Corrosion detection sensor device

ABSTRACT

A sensor device includes a first electrode, a second electrode and a functional element. The first electrode includes a porous body having a connecting hole where adjacent holes communicate with each other with the porous body being in at least the vicinity of a surface of the first electrode. The second electrode is spaced apart from the first electrode. The functional element is configured to measure a difference in electric potential between the first electrode and the second electrode. The sensor device is configured to measure a state of a site to be measured based on the difference in electric potential as measured by the functional element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2011-107934 filed on May 13, 2011. The entire disclosure of JapanesePatent Application No. 2011-107934 is hereby incorporated herein byreference.

BACKGROUND

1. Technical Field

The present invention relates to a sensor device.

2. Related Art

There are known sensor devices which, for example, measure the state ofcorrosion of a reinforcing bar in concrete (e.g., see Japanese Laid-OpenPatent Publication 6-222033).

Typically, the concrete in a concrete structure immediately afterconstruction exhibits a strong alkalinity. For this reason, thereinforcing bars in a concrete structure immediately after constructionhave a passivation film formed on the surface thereof and are thereforesafe. However, in concrete structure that is affected after constructionby acid rain, exhaust gas, and the like, the concrete will be graduallyacidified, and the reinforcing bars will therefore corrode.

For example, in the device recited in the above mentioned publication, aprobe provided with a reference electrode and a counter electrode isembedded in concrete and measures the polarization resistance andchanges in electric potential caused by the corrosion of the reinforcingbars, whereby the corrosion of the reinforcing bars is predicted.

In such a device, the reference electrode and the counter electrodeembedded in the concrete are used to measure the self-potential of thereinforcing bars, which are used as a working electrode, but a corrosionresponse does not progress when the surfaces of the reinforcing bars donot have sufficient moisture. For this reason, in some cases, when thesurfaces of the reinforcing bars do not have sufficient moisture, eventhough a reinforcing bar may have a corroded region, no difference(gradient) in electric potential between the corroded region and thenon-corroded region occurs. In view of such a fact, in the devicerecited in the above mentioned publication, a fluctuation in moistureinside the concrete has a major impact and there is a variance to theself-potential (gradient) of the reinforcing bars, thus rendering itdifficult to accurately predict the corrosion of the reinforcing bars.

SUMMARY

An objective of the present invention is to provide a sensor device withwhich it is possible, after reinforcing bars have been constructed, tomeasure changes in the state of an object to be measured during theperiod up until corrosion begins, and to use the resulting informationin planning the preservation of the concrete structure.

Such an objective is achieved by the present invention described below.

A sensor device according to one aspect of the present inventionincludes a first electrode, a second electrode and a functional element.The first electrode includes a porous body having a connecting holewhere adjacent holes communicate with each other with the porous bodybeing in at least the vicinity of a surface of the first electrode. Thesecond electrode is spaced apart from the first electrode. Thefunctional element is configured to measure a difference in electricpotential between the first electrode and the second electrode. Thesensor device is configured to measure a state of a site to be measuredbased on the difference in electric potential as measured by thefunctional element.

According to the sensor device having such a configuration, the surfacearea of the first electrode can be increased because the connectingholes (fine pores) open onto the surface of the first electrode. Forthis reason, the amount of moisture adhering to the first electrode canbe increased.

Further, the capillary condensation effect originating from theconnecting holes (fine holes) opening on the surface of the firstelectrode makes it possible to cause moisture to condense on the firstelectrode at a lower relative humidity. For this reason, a stablepresence of liquid water can be maintained on the first electrode.

In view of such a fact, a fluctuation in the amount of moisture on thefirst electrode can be prevented even though the relative humidity ofthe site to be measured may change in association with changes in thehumidity or temperature of the external environment. Consequently,changes in the humidity or temperature of the external environment canbe prevented from causing the self-potential of the first electrode tofluctuate, and the state of the site to be measured can be measured witha high degree of precision.

In the sensor device according to the above described aspect of thepresent invention, the second electrode preferably includes a porousbody having a connecting hole where adjacent holes communicate with eachother with the porous body being in at least the vicinity of a surfaceof the second electrode.

This makes it possible to prevent a fluctuation in the amount ofmoisture on the second electrode even though the relative humidity ofthe site to be measured may change in association with changes in thehumidity or temperature of the external environment. Consequently,changes in the humidity or temperature of the external environment canbe prevented from causing the self-potential of the second electrode tofluctuate, and the state of the site to be measured can be measured witha high degree of precision.

In the sensor device according to the above described aspect of thepresent invention, the first electrode preferably includes a firstmetallic material in which either a first passivation film is formed ona surface thereof or a first passivation film present on a surfacethereof is lost, in association with changes in an environment of thesite to be measured.

Thereby, the difference in electric potential between the firstelectrode and the second electrode has sharp changes depending on thepresence or absence of the first passivation film as associated withchanges in the pH of the site to be measured. For this reason, it ispossible to accurately measure whether or not the pH of the site to bemeasured is at or below a set value.

The difference in electric potential between the first electrode and thesecond electrode also has sharp changes depending on the loss of thefirst passivation film, which is associated with a change in thechloride ion concentration of the site to be measured. For this reason,it is possible to accurately measure whether or not the chloride ionconcentration of the site to be measured is at or below a set value.

In the sensor device according to the above described aspect of thepresent invention, the second electrode preferably includes a secondmetallic material in which either a second passivation film is formed ona surface thereof or a second passivation film present on a surfacethereof is lost, in association with changes in the environment of thesite to be measured.

Thereby, the difference in electric potential between the firstelectrode and the second electrode has sharp changes depending on thepresence or absence of the second passivation film as associated withchanges in the pH of the site to be measured. For this reason, it ispossible to accurately measure whether or not the pH of the site to bemeasured is at or below a set value.

The difference in electric potential between the first electrode and thesecond electrode also has sharp changes depending on the loss of thesecond passivation film, which is associated with a change in thechloride ion concentration of the site to be measured. For this reason,it is possible to accurately measure whether or not the chloride ionconcentration of the site to be measured is at or below a set value.

In the sensor device according to the above described aspect of thepresent invention, each of the first metallic material and the secondmetallic material is preferably iron or an iron-based alloy.

Iron or iron-based alloys (iron-based materials) are more readily andmore inexpensively procured. In a case where, for example the sensordevice is used to measure the state of a concrete structure, then atleast one electrode of the first electrode and the second electrode canbe constituted of the same material as the reinforcing bars inside theconcrete structure, and it is possible to effectively detect the stateof corrosion of the reinforcing bars inside the concrete structure.

In the sensor device according to the above described aspect of thepresent invention, at least one of the first electrode and the secondelectrode preferably includes a substrate and a conductive film providedon the substrate including a material different from that of thesubstrate.

This makes it possible for the vicinity of the surface of at least oneelectrode or more of the first electrode and the second electrode to beconstituted of a metal from which it is difficult to produce a porousbody. It is also possible to use the thickness of the conductive film toadjust the diameter of the holes of the porous body constituting the atleast one electrode or more of the first electrode and the secondelectrode.

In the sensor device according to the above described aspect of thepresent invention, preferably, the conductive film preferably includes ametallic material in which either a passivation film is formed on asurface thereof or a passivation film present on a surface thereof islost, in association with changes in an environment of the site to bemeasured.

Thereby, the difference in electric potential between the firstelectrode and the second electrode has sharp changes depending on thepresence or absence of the passivation film as associated with changesin the pH of the site to be measured. For this reason, it is possible toaccurately measure whether or not the pH of the site to be measured isat or below a set value.

The difference in electric potential between the first electrode and thesecond electrode also has sharp changes depending on the loss of thepassivation film, which is associated with a change in the chloride ionconcentration of the site to be measured. For this reason, it ispossible to accurately measure whether or not the chloride ionconcentration of the site to be measured is at or below a set value.

In the sensor device according to the above described aspect of thepresent invention, the functional element is preferably furtherconfigured to detect whether or not pH or chloride ion concentration atthe site to be measured is at or below a set value, based on thedifference in electric potential between the first electrode and thesecond electrode.

This makes it possible to detect the changes in state of an object to bemeasured which accompany changes in the pH or chloride ion concentrationthereof.

The sensor device according to the above described aspect of the presentinvention preferably further includes an antenna and a communicationcircuit configured to provide power to the antenna, and the functionalelement is preferably further configured to drive and control thecommunication circuit.

This makes it possible to wirelessly transmit measurement results to theoutside of the object to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a drawing illustrating an example of the state of use of asensor device according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of thesensor device illustrated in FIG. 1.

FIG. 3 is a plan view of a first electrode, a second electrode, and afunctional element illustrated in FIG. 2.

FIG. 4 is a cross-sectional view (a cross-sectional view along line A-Ain FIG. 3) for describing the first electrode and the second electrodeillustrated in FIG. 2.

FIG. 5 is a cross-sectional view (a cross-sectional view along line B-Bin FIG. 3) for describing the functional element illustrated in FIG. 2.

FIG. 6A is an enlarged sectional view illustrating an example of aconfiguration of the first electrode illustrated in FIG. 2, and FIG. 6Bis an enlarged sectional view illustrating an example of a configurationof the second electrode illustrated in FIG. 2.

FIG. 7 is a circuit diagram illustrating a differential amplifiercircuit provided to the functional element illustrated in FIG. 2.

FIG. 8 is a circuit diagram illustrating the differential amplifiercircuit provided to the functional element illustrated in FIG. 2.

FIG. 9A is a drawing illustrating an example of the manner in which thepH and electric potential of iron is related to the state, and FIG. 9Bis a drawing illustrating an example of the manner in which the pH andelectric potential of iron-aluminum is related to the state.

FIG. 10 is a drawing for describing an example of the action of thesensor device illustrated in FIG. 1.

FIG. 11 is a drawing illustrating an example of the state of use of asensor device according to a second embodiment of the present invention.

FIG. 12A is an enlarged sectional view illustrating an example of aconfiguration of the first electrode illustrated in FIG. 11, and FIG.12B is an enlarged sectional view illustrating an example of aconfiguration of the second electrode illustrated in FIG. 11.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following is a description of preferred embodiments of the sensordevice of the present invention, with reference to the accompanyingdrawings.

First Embodiment

The first embodiment of the present invention shall be described first.

FIG. 1 is a drawing illustrating an example of the state of use of asensor device according to a first embodiment of the present invention.FIG. 2 is a block diagram illustrating a schematic configuration of thesensor device illustrated in FIG. 1. FIG. 3 is a plan view of a firstelectrode, a second electrode, and a functional element illustrated inFIG. 2. FIG. 4 is a cross-sectional view (a cross-sectional view alongline A-A in FIG. 3) of the first electrode and the second electrodeillustrated in FIG. 2. FIG. 5 is a cross-sectional view (across-sectional view along line B-B in FIG. 3) of the functional elementillustrated in FIG. 2. FIG. 6A is an enlarged sectional viewillustrating an example of a configuration of the first electrodeillustrated in FIG. 2. FIG. 6B is an enlarged sectional viewillustrating an example of a configuration of the second electrodeillustrated in FIG. 2. FIGS. 7 and 8 are each circuit diagramsillustrating a differential amplifier circuit provided to the functionalelement illustrated in FIG. 2. FIG. 9A is a drawing illustrating anexample of the manner in which the pH and electric potential of iron isrelated to the state. FIG. 9B is a drawing illustrating an example ofthe manner in which the pH and electric potential of iron-aluminum isrelated to the state. FIG. 10 is a drawing for describing an example ofthe action of the sensor device illustrated in FIG. 1.

The example described below is that of a case where the sensor device ofthe present invention is used to measure the quality of a concretestructure.

A sensor device 1 shown in FIG. 1 is intended to measure the quality ofa concrete structure 100.

The concrete structure 100 has a plurality of reinforcing bars 102embedded in concrete 101. The sensor device 1 is also embedded withinthe concrete 101 of the concrete structure 100, in the vicinity of thereinforcing bars 102. The sensor device 1 may be embedded when theconcrete structure 100 is being cast, prior to the casting of theconcrete 101, so as to be fixed to the reinforcing bars 102, or may beembedded in holes bored into the concrete 101 having hardened aftercasting.

The sensor device 1 has a main body 2, as well as a first electrode 3and a second electrode 4 exposed to the surface of the main body 2. Inthe present embodiment, the first electrode 3 and the second electrode 4are installed on the outer surface of the concrete structure 100 furtherout than the reinforcing bars 102 so that both are equidistant from theouter surface of the concrete structure 100. The first electrode 3 andthe second electrode 4 are also installed such that the respectiveelectrode surfaces thereof are parallel or substantially parallel to theouter surface of the concrete structure 100. The first electrode 3 andthe second electrode 4 are also configured such that the difference inelectric potential therebetween changes in association with changes inpH of a site to be measured of the concrete 101. More detaileddescriptions of the first electrode 3 and the second electrode 4 shallbe provided below.

The sensor device 1, as illustrated in FIG. 2, also has a functionalelement 51, a power source 52, a temperature sensor 53, a communicationcircuit 54, an antenna 55, and an oscillator 56, which are electricallyconnected to the first electrode 3 and to the second electrode 4 and arehoused within the main body 2.

The following is a sequential description of each of the partsconstituting the sensor device 1.

Main Body

The main body 2 has a function for supporting the first electrode 3, thesecond electrode 4, the functional element 51, and other elements.

Such a main body 2, as illustrated in FIG. 4 and FIG. 5, has a substrate21 for supporting the first electrode 3, the second electrode 4, and thefunctional element 51. The substrate 21 is also intended to support thepower source 52, the temperature sensor 53, the communication circuit54, the antenna 55, and the oscillator 56, but FIGS. 3 to 5 omit adepiction of the power source 52, the temperature sensor 53, thecommunication circuit 54, the antenna 55, and the oscillator 56, forconvenience of description.

The substrate 21 has insulating properties. Examples which can be usedas the substrate 21 include, but are not particularly limited to, analumina substrate, a resin substrate, or the like.

An insulating layer 23 constituted of an insulating resin composition,such as, for example, a solder resist, is provided on the substrate 21.The first electrode 3, the second electrode 4, and the functionalelement 51 are also mounted onto the substrate 21 via the insulatinglayer 23.

As illustrated in FIG. 5, the functional element 51 (an integratedcircuit chip) is retained on the substrate 21, and conductor parts 61,62 (an electrode pad) of the functional element 51 are connected to thefirst electrode 3 and the second electrode 4.

The conductor part 61 electrically connects the first electrode 3 withconductor parts 516 a, 516 d as well as with a gate electrode of atransistor 514 a. The conductor part 62 electrically connects the secondelectrode 4 with conductor parts 516 b, 516 e as well as with a gateelectrode of a transistor 514 b. Each of the first electrode 3 and thesecond electrode 4 is in a floating state because of the respectiveconnections thereof with the gate electrodes of the transistors 514 a,514 b. Reference numerals 515 a and 515 b indicate interlayer insulatingfilms of the integrated circuit, and reference numeral 25 indicates aprotective film of the integrated circuit.

The main body 2 also has a function for housing the functional element51, the power source 52, the temperature sensor 53, the communicationcircuit 54, the antenna 55, and the oscillator 56.

In particular, the main body 2 is configured so as to provide aliquid-tight housing for the functional element 51, the power source 52,the temperature sensor 53, the communication circuit 54, the antenna 55,and the oscillator 56.

Specifically, as illustrated in FIGS. 4 and 5, the main body 2 has asealing part 24. The sealing part 24 has a function for sealing in thefunctional element 51, the power source 52, the temperature sensor 53,the communication circuit 54, the antenna 55, and the oscillator 56.This makes it possible to prevent the deterioration of the functionalelement 51, the power source 52, the temperature sensor 53, thecommunication circuit 54, the antenna 55, and the oscillator 56 in acase where the sensor device 1 is installed in the presence of moistureor concrete.

Herein, the sealing part 24 has an opening part 241, and is providedsuch that each of the parts other than the first electrode 3 and thesecond electrode 4 are covered, while the first electrode 3 and thesecond electrode 4 are exposed from the opening part 241 (see FIGS. 3and 4). This makes it possible for the sensor device 1 to measure whilethe sealing part 24 prevents each of the parts other than the firstelectrode 3 and the second electrode 4 from deteriorating. The openingpart 241 may also be formed such that at least a part or more of thefirst electrode 3 and at least a part or more of the second electrode 4is exposed.

Examples of materials which can be used to constitute the sealing part24 include: a thermoplastic resin, such as an acrylic-based resin, aurethane-based resin, or an olefin-based resin; a thermosetting resin,such as an epoxy-based resin, a melamine-based resin, or a phenol-basedresin; and various other types of resin materials, it being possible touse one type thereof or a combination of two or more types thereof.

The sealing part 24 may be provided or can be omitted, in accordancewith need.

First Electrode and Second Electrode

The first electrode 3 and the second electrode 4, as illustrated in FIG.4, are each provided on the outer surface of the main body 2 describedabove (more specifically, on the substrate 21). In particular, the firstelectrode 3 and the second electrode 4 are provided on the same plane.For this reason, it is possible to prevent the emergence of differencesin the installation environments of the first electrode 3 and the secondelectrode 4.

The first electrode 3 and the second electrode 4 are spaced apart tosuch an extent (for example, several millimeters) that there is nomutual influence due to electric potential.

In the present embodiment, each of the first electrode 3 and the secondelectrode 4 forms the shape of a thin film. Each of the shapes in planview of the first electrode 3 and the second electrode 4 also forms aquadrangle. The first electrode 3 and the second electrode 4 havemutually equivalent shapes and surface areas in plan view.

In particular, the first electrode 3, as illustrated in FIG. 6A, isconstituted of a porous body 32 having a plurality of holes 31. Theplurality of holes 31 form connecting holes (fine pores), where adjacentholes 31 communicate with each other, and the connecting holes provideopenings on the surface of the first electrode 3.

Similarly, the second electrode 4, as illustrated in FIG. 6B, isconstituted of a porous body 42 having a plurality of holes 41. Theplurality of holes 41 form connecting holes (fine pores), where adjacentholes 41 communicate with each other, and the connecting holes provideopenings on the surface of the second electrode 4.

Such connecting holes make it possible to give each of the firstelectrode 3 and the second electrode 4 a greater surface area. For thisreason, the amount of moisture adhering to each of the first electrode 3and the second electrode 4 can be increased.

The capillary condensation effect endowed by the fine pores makes itpossible to cause moisture to condense on each of the first electrode 3and the second electrode 4 at a lower relative humidity. For thisreason, the presence of liquid water on each of the first electrode 3and on the second electrode 4 can be rendered stable. Specifically, thepresence of liquid water can be ensured through condensation on each ofthe first electrode 3 and the second electrode 4 even at a low relativehumidity where condensation would not form on the first electrode 3 andon the second electrode 4 in a hypothetical case where the firstelectrode 3 and the second electrode 4 are constituted of compactbodies.

In view of such a fact, a fluctuation in the amount of moisture on thefirst electrode 3 and on the second electrode 4 can be prevented eventhough the relative humidity inside the concrete 101 may change inassociation with changes in the humidity or temperature of the externalenvironment. Consequently, changes in the humidity or temperature of theexternal environment can be prevented from causing the self-potential ofthe first electrode 3 and the second electrode 4 to fluctuate, and thestate of the site to be measured of the concrete 101 can be measuredwith a high degree of precision.

Preferably, the average diameter of the plurality of holes 31 and theplurality of holes 41 is, for example, 2 nm to 50 nm, but there is noparticular limitation thereto, provided that the range thereof allowsfor the occurrence of the capillary condensation effect as describedabove. That is, preferably, the holes 31 and the holes 41 are mesopores.Also, the average diameter of the plurality of holes 31 and the averagediameter of the plurality of holes 41 may be mutually identical ordifferent.

Preferably, the porosity of each of the first electrode 3 and of thesecond electrode 4 is, for example, 10% to 90%, but there is noparticular limitation thereto, provided that the range thereof allowsfor the occurrence of the capillary condensation effect as describedabove. The porosity of the first electrode 3 and the porosity of thesecond electrode 4 may be mutually identical or different.

In the present embodiment, the first electrode 3 and the secondelectrode 4 are constituted of mutually different materials. Thefollowing is a more detailed description of the constituent materials ofthe first electrode 3 and the second electrode 4.

Such a first electrode 3 is constituted of a first metallic material(which hereinafter is also simply called the “first metallic material”)for forming a passivation film (a first passivation film). In the firstelectrode 3 having such a configuration, a passivation film is eitherformed or destroyed depending on changes in the pH. In the state wherethe passivation film has been so formed (the passivated state) on thefirst electrode 3, inactive (noble) conditions are in effect andself-potential increases (a shift towards increased nobility occurs). Inthe state where the passivation film has been destroyed (the state wherethe passivation film has been lost), the first electrode 3 is active (ofless nobility). For this reason, the electric potential of the firstelectrode 3 has sharp changes depending on the presence or absence ofthe passivation film, as associated with changes in pH.

The first metallic material is not particularly limited, provided that apassivation film is formed; examples thereof include iron, nickel,magnesium, zinc, an alloy containing these elements, or the like.

For example, iron forms a passivation film when the pH is greater than 9(see FIG. 9A). Iron-aluminum-based carbon steel (0.8% Al) also forms apassivation film when the pH is greater than 4 (see FIG. 9B). Nickelforms a passivation film when the pH is 8 to 14. Magnesium forms apassivation film when the pH is greater than 10.5. Zinc forms apassivation film when the pH is 6 to 12.

Of these, the first metallic material is preferably iron or an alloycontaining iron (an iron-based alloy), i.e., an iron-based material(specifically, carbon steel, alloy steel, SUS, and the like). Iron-basedmaterials are comparatively more readily and more inexpensivelyprocured. In a case where, as in the present embodiment, the sensordevice 1 is used to measure the state of the concrete structure 100,then the first metallic material can be a material identical to orapproximating that of the reinforcing bars 102 of the concrete structure100, and it is possible to effectively detect a state of a corrosiveenvironment of the reinforcing bars 102. In the case where, for example,the first electrode 3 is constituted of iron, then a determination canbe made as to whether or not the pH is 9 or greater.

On the other hand, the second electrode 4 is constituted of a secondmetallic material different from the first metallic material (whichhereinafter is also simply called “the second metallic material”). Apassivation film of the second electrode 4 having such a configurationis neither formed nor destroyed (lost), nor is there any sharp change inelectric potential, when the electrode potential of the first electrode3 changes depending on the presence or absence of the passivation film,as described above. For this reason, the difference in electricpotential between the first electrode 3 and the second electrode 4 hassharp changes when the electric potential of the first electrode 3changes depending on the presence or absence of the passivation film asdescribed above. For this reason, it is possible to accurately detectwhether or not the pHs of the installation environments of the firstelectrode 3 and the second electrode 4 (which, in this embodiment, arein the vicinity of the reinforcing bars 102 of the concrete 101) are ator below a set value.

Various types of metallic materials can be used as the second metallicmaterial without particular limitation, provided that it is a metallicmaterial in which the pH dependency with respect to the formation and/orloss of the passivation film is different from that of the firstmetallic material.

The second metallic material, with the provision of being a differentmetallic material from the aforesaid first metallic material, may form apassivation film or may not form a passivation film.

In a case where the second metallic material does form a passivationfilm (the second passivation film), then metals which can serve as thesecond metallic material include those examples provided for the firstmetallic material.

A preferred aspect of the present invention is that a first pH and asecond pH are mutually different, where the first pH (a firstpassivation pH) is the lower limit of the range of pHs in which thefirst metallic material forms a passivation film, and the second pH (asecond passivation pH) is the lower limit of the range of pHs in whichthe second metallic material forms a passivation film. That is, thefirst metallic material forms a passivation film when the pH thereofbecomes greater than the first pH, and the second metallic materialforms a passivation film when the pH thereof becomes greater than thesecond pH, which is different from the first pH. This makes it possibleto accurately and respectively detect whether or not the pHs in theenvironments where the first electrode 3 has been installed and wherethe second electrode 4 has been installed are the first pH or lower orare the second pH or lower.

In such a case, preferably, the first pH is 8 to 10, and the second pHis 7 or lower. This also makes it possible, by detecting whether or notthe pH is at or lower than the first pH, to know in advance that theinstallation environments of the first electrode 3 and the secondelectrode 4 are approaching a neutral state. In view of such facts, in acase where the sensor device 1 is used to measure the state of theconcrete structure 100, as in this embodiment, it is possible to act inadvance to counter and prevent the corrosion of the reinforcing bars102. It is also possible, by detecting whether or not the pH is at orlower than the second pH, to know that the installation environments ofthe first electrode 3 and the second electrode 4 have reached an acidicstate.

In such a case, preferably, the second metallic material is iron or analloying containing iron (an iron-based alloy), i.e., an iron-basedmaterial. Iron-based materials are comparatively more readily and moreinexpensively procured. Further, in a case where the sensor device 1 isused to measure the state of the concrete structure 100, as in thisembodiment, then it is possible for the second metallic material to bethe same material as the reinforcing bars 102. Having the secondmetallic material be the same material as the reinforcing bars 102 makesit possible to effectively detect the state of corrosion of thereinforcing bars 102.

On the other hand, in a case where the second metallic material does notform a passivation film, then possible examples of the second metallicmaterial include platinum, gold, and the like. In a case where thesecond metallic material does not form a passivation film, then it ispossible to know in a single stage, with a high degree of precision, thechange when the installation environments of the first electrode 3 andthe second electrode 4 change from a strongly alkaline state to astrongly acidic state.

In such a case, preferably, the first metallic material forms apassivation film when the pH thereof becomes greater than a pH of 3 to5, or, greater than a pH of 8 to 10. It is possible, by detectingwhether or not the pH is a pH of at or lower than a pH of 3 to 5, toknow that the installation environments of the first electrode 3 and thesecond electrode 4 have reached an acidic state. Detecting whether ornot the pH is at or below a pH of 8 to 10 also makes it possible to knowin advance that the installation environments of the first electrode 3and the second electrode 4 are approaching a neutral state.

According to another aspect of the present invention, a case where thesecond metallic material does form a passivation film involves a firstchloride ion concentration and a second chloride ion concentration,which are mutually different, where the first chloride ion concentrationis the lower limit of the chloride ion concentration at which thepassivation film of the first metallic material begins to be destroyedand the second chloride ion concentration is the lower limit of thechloride ion concentration at which the passivation film of the secondmetallic material begins to be destroyed. That is, the passivation filmof the first metallic material begins to be destroyed when the chlorideion concentration becomes greater than the first chloride ionconcentration, and the second metallic material begins to disintegratewhen the chloride ion concentration becomes greater than the secondchloride ion concentration. This makes it possible to accurately andrespectively detect whether or not the chloride ion concentrations ofthe environments where the first electrode 3 has been installed andwhere the second electrode 4 has been installed are at or below thefirst chloride ion concentration or are at or below the second chlorideion concentration.

In view of such facts, in the case where the sensor device 1 is used tomeasure the state of the concrete structure 100, as in this embodiment,then it is possible to detect CO₂ (neutralization) and chloride ionsinfiltrating into the concrete from outside, before the reinforcing bars102 inside the concrete structure 100 are reached. Accordingly, it ispossible to act to counter and prevent corrosion before the reinforcingbars 102 are corroded.

Each of such a first electrode 3 and a second electrode 4 is notparticularly limited and can be formed by a known method for forming aporous body film. The shapes of the plurality of holes 31 and theplurality of holes 41 illustrated in FIG. 6 are each one example, therebeing no limitation to what has been depicted, provided that the firstelectrode 3 and the second electrode 4 be able to exert the capillarycondensation effect as described above. The first electrode 3 and thesecond electrode 4 can be constituted of various known porous bodieshaving connecting holes.

Functional Element

The functional element 51 is embedded in the interior of the aforesaidmain body 2. The surface of the substrate 21 of the main body 2 to whichthe functional element 51 is provided may be identical to or oppositefrom that of the first electrode 3 and the second electrode 4.

The functional element 51 has a function for measuring the difference inelectric potential between the first electrode 3 and the secondelectrode 4. This makes it possible to detect whether or not the pHs ofthe installation environments of the first electrode 3 and the secondelectrode 4 are at or below a set value, based on the difference inelectric potential between the first electrode 3 and the secondelectrode 4.

The functional element 51 also has a function for detecting whether ornot the pH or chloride ion concentration of the site to be measured ofthe concrete structure 100, which is the object to be measured, is at orbelow a set value, based on the difference in electric potential betweenthe first electrode 3 and the second electrode 4. This makes it possibleto detect a change in state of the concrete structure 100 in associationwith a change in the pH or a change in the chloride ion concentrationthereof.

Such a functional element 51 is, for example, an integrated circuit.More specifically, the functional element 51 is, for example, an MCU (amicro control unit) and has, as illustrated in FIG. 2, a CPU 511, an A/Dconversion circuit 512, and a differential amplifier circuit 514.

A more specific description shall now be provided. The functionalelement 51, as illustrated in FIG. 5, has: a substrate 513; a pluralityof transistors 514 a, 514 b, 514 c provided on the substrate 513;interlayer insulating films 515 a, 515 b for covering the transistors514 a, 514 b, 514 c; conductor parts 516 a, 516 b, 516 c, 516 d, 516 e,516 f constituting a wiring and a conductor post; a protective film 25;and conductor parts 61, 62 constituting an electrode pad.

The substrate 513 is, for example, an SOI substrate, on which the CPU511 and the A/D conversion circuit 512 are formed. Using an SOIsubstrate as the substrate 513 makes it possible to make the transistors514 a to 514 c into an SOI-type MOSFET.

The plurality of transistors 514 a, 514 b, 514 c are each, for example,field-effect transistors (FETs), and constitute a part of thedifferential amplifier circuit 514.

The differential amplifier circuit 514, as illustrated in FIG. 7, isconstituted of the three transistors 514 a to 514 c as well as a currentmirror circuit 514 d.

The differential amplifier circuit 514 also has operating amplifiers201, 202 and an operating amplifier 203, as illustrated in FIG. 8.

The operating amplifier 201 detects the electric potential of the firstelectrode 3 using a comparative electrode 7 as a reference. Theoperating amplifier 202 detects the electric potential of the secondelectrode 4 using the comparative electrode 7 as a reference. Theoperating amplifier 203 detects the difference between the outputtedelectric potential of the operating amplifier 201 and the outputtedelectric potential of the operating amplifier 202.

The conductor part 516 a has one end connected to a gate electrode ofthe transistor 514 a, and another end connected to the aforesaidconductor part 516 d. The conductor part 516 d is electrically connectedto the first electrode 3 via the conductor part 61. An electricalconnection is thereby formed between the first electrode 3 and the gateelectrode of the transistor 514 a. For this reason, the drain current ofthe transistor 514 a changes in accordance with changes in the electricpotential of the first electrode 3.

Similarly, the conductor part 516 b has one end connected to a gateelectrode of the transistor 514 b, and another end connected to theaforesaid conductor part 516 e. The conductor part 516 e is electricallyconnected to the second electrode 4 via the conductor part 62. Anelectrical connection is thereby formed between the second electrode 4and the gate electrode of the transistor 514 b. For this reason, thedrain current of the transistor 514 b changes in accordance with changesin the electric potential of the second electrode 4.

The conductor part 516 c has one end connected to a gate electrode ofthe transistor 514 c, and another end connected to the aforesaidconductor part 516 f, thus constituting a part of a circuit.

The functional element 51 is operated by energization from the powersource 52. Provided that the power source 52 can supply electric powercapable of operating the functional element 51, there is no particularlimitation, and the power source 52 may be, for example, a battery suchas a button-type battery, or may be a power source using an elementhaving a power generation function, such as a piezoelectric element.

The functional element 51 is configured so as to be able to acquiredetected temperature information on the temperature sensor 53. Thismakes it possible also to obtain information relating to the temperatureof the site to be measured. The use of such information relating to thetemperature makes it possible to more accurately measure the state ofthe site to be measured, or to anticipate changes in the site to bemeasured with a high degree of precision.

The temperature sensor 53 has a function for detecting the temperatureof the site to be measured of the concrete structure 100, which is theobject to be measured. Examples of temperature sensors which can be usedas such a temperature sensor 53 include but are not particularly limitedto a thermocouple or other various known types.

The functional element 51 also has a function for driving andcontrolling the communication circuit 54. For example, the functionalelement 51 respectively inputs, into the communication circuit 54,information relating to the difference in electric potential between thefirst electrode 3 and the second electrode 4 (which hereinafter is alsosimply called “electric potential difference information”) as well asinformation relating to whether or not the pH or chloride ionconcentration of the site to be measured is at or below a set value(which hereinafter is also simply called “pH information”). Thefunctional element 51 also additionally inputs, into the communicationcircuit 54, information relating to the temperature detected by thetemperature sensor 53 (which hereinafter is also simply called“temperature information”).

The communication circuit 54 has a function for supplying power to theantenna 55 (a transmitting function). This makes it possible for thecommunication circuit 54 to wirelessly transmit inputted information viathe antenna 55. The transmitted information is received by a receiver(reader) provided outside the concrete structure 100.

The communication circuit 54 has, for example, a transmission circuitfor transmitting electromagnetic waves, a modulation circuit having afunction for modulating a signal, and the like. The communicationcircuit 54 may also have a down converter circuit having a function forconverting a signal to a lower frequency, an up converter having afunction for converting a signal to a higher frequency, an amplifiercircuit having a function for amplifying a signal, a receiving circuitfor receiving electromagnetic waves, a demodulating circuit having afunction for demodulating a signal, and the like.

The antenna 55 is constituted of for example, a metallic material,carbon, or the like, but is not particularly limited thereto, and formsa winding wire, a thin film, or another form.

The functional element 51 is configured so as to be able to acquire aclock signal from the oscillator 56. This makes it possible tosynchronize each of the circuits, or to add time information to each ofthe various forms of information.

The oscillator 56 is constituted of, for example, an oscillation circuitemploying a crystal oscillator, but is not particularly limited thereto.

In a measurement method using the sensor device 1 configured as has beendescribed above, the first electrode 3 and the second electrode 4 areeach embedded in the concrete structure 100, which is the object to bemeasured, and the state of the concrete structure 100 is measured basedon the difference in electric potential between the first electrode 3and the second electrode 4.

The following is a description of the action of the sensor device 1using, by way of example, a case where the first electrode 3 isconstituted of iron and the second electrode 4 is constituted ofiron-aluminum.

In the concrete structure 100 immediately after casting, ordinarily, theconcrete 101 exhibits a strong alkalinity when casting has been doneappropriately. For this reason, at such a time, the first electrode 3and the second electrode 4 each forms stable passivation films, asillustrated in FIGS. 9A and 9B. That is, as illustrated in FIG. 10A, apassivation film 33 is formed on the surface of the first electrode 3,and a passivation film 43 is formed on the surface of the secondelectrode 4. The self-potentials of the first electrode 3 and the secondelectrode 4 are thereby each made to increase (become more noble). As aresult, the difference in electric potential between the first electrode3 and the second electrode 4 immediately after the concrete has beencast is reduced.

Thereafter, the pH of the concrete 101 in the concrete structure 100gradually changes toward becoming acidic due to the effects of carbondioxide, acidic rain, exhaust gas, and the like.

When the pH of the concrete 101 drops to as low as about 9, then, asillustrated in FIG. 10B, although the passivation film 43 of the secondelectrode 4 is stable and the self-potential thereof changes onlyslightly, the passivation film of the first electrode 3 begins todisintegrate, and thus the self-potential thereof drops (becomes lessnoble). The difference in electric potential between the first electrode3 and the second electrode 4 is thereby increased.

When the pH of the concrete 101 drops to as low as about 4, then, asillustrated in FIG. 10C, the passivation film of the second electrode 4also begins to disintegrate, and the self-potential thereof drops. Atsuch a time, because the self-potentials of both the first electrode 3and the second electrode 4 drop, the difference in electric potentialbetween the first electrode 3 and the second electrode 4 is once againreduced. At such a time, each of the first electrode 3 and the secondelectrode 4 is undergoing progressive corrosion.

Thus, the difference in electric potential between the first electrode 3and the second electrode 4 has sharp changes at two different times,which are the time when the pH reaches about 9 and the time when the pHreaches about 4. For this reason, it is possible to respectively detectwith a high degree of precision that the pH of the site to be measuredhas reached about 9, and that the pH of the site to be measured hasreached about 4.

The use of such detection results makes it possible to monitor for along time the temporal changes in the qualities of the concretestructure 100 after casting. For this reason, it is possible to becomeaware of the deterioration of the concrete 101 (neutralization or theintrusion of saline matter) before the reinforcing bars 102 arecorroded. This makes it possible to paint the concrete structure 100 orperform repair work by a mixed anti-corrosion agent mortar or the like,before the reinforcing bars 102 are corroded.

It is also possible to determine whether or not there has been anyabnormality during the casting of the concrete structure 100. For thisreason, it is possible to prevent initial difficulties with the concretestructure 100, and to improve the quality of the concrete structure 100.

According to the sensor device 1 of the first embodiment as has beendescribed above, because each of the first electrode 3 and the secondelectrode 4 is constituted of a porous body having connecting holes,each of the first electrode 3 and the second electrode 4 can be given agreater surface area. For this reason, the amount of moisture adheringto each of the first electrode 3 and the second electrode 4 can beincreased.

The capillary condensation effect endowed by the connecting holes (finepores) of the first electrode 3 and of the second electrode 4 makes itpossible to cause moisture to condense on each of the first electrode 3and the second electrode 4 at a lower relative humidity. For thisreason, a stable presence of liquid water can be maintained on the firstelectrode 3 and on the second electrode 4.

In view of such a fact, a fluctuation in the amount of moisture on thefirst electrode 3 and on the second electrode 4 can be prevented eventhough the relative humidity inside the concrete 101 may change inassociation with changes in the humidity or temperature of the externalenvironment. Consequently, changes in the humidity or temperature of theexternal environment can be prevented from causing the self-potential ofthe first electrode 3 and the second electrode 4 to fluctuate, and thestate of the site to be measured of the concrete 101 can be measuredwith a high degree of precision.

Second Embodiment

The following is a description of a second embodiment of the presentinvention.

FIG. 11 is a drawing illustrating an example of the state of use of asensor device according to a second embodiment of the present invention.FIG. 12A is an enlarged sectional view illustrating an example of aconfiguration of the first electrode illustrated in FIG. 11.

FIG. 12B is an enlarged sectional view illustrating an example of aconfiguration of the second electrode illustrated in FIG. 11.

The following description of the second embodiment focuses on the pointsof difference with the embodiment described above, and omits adescription of any similar matters.

The sensor device of the second embodiment is substantially similar tothe sensor device of the first embodiment, except in that the shapes inplan view and number of the first electrode and the second electrode aredifferent, and in that the structure of the first electrode isdifferent. Constituent elements which are similar to the embodimentdescribed above have been assigned like reference numerals.

A sensor device sensor device 1A of this embodiment has a main body 2A,as well as a plurality of first electrodes 3 a, 3 b, 3 c and a pluralityof second electrodes 4 a, 4 b, 4 c exposed to the surface of the mainbody 2A.

In this embodiment, the first electrodes 3 a, 3 b, 3 c and the secondelectrodes 4 a, 4 b, 4 c are provided mutually spaced apart. Also, thefirst electrodes 3 a, 3 b, 3 c and the second electrodes 4 a, 4 b, 4 care each installed such that the electrode surface becomes perpendicularto or substantially perpendicular to the outer surface of the concretestructure 100.

The plurality of first electrodes 3 a, 3 b, 3 c are all at mutuallydifferent distances from the outer surface of the concrete structure100. Specifically, the plurality of first electrodes 3 a, 3 b, 3 c areprovided lined up in the stated order, from the outer surface of theconcrete structure 100 inward.

Similarly, the plurality of second electrodes 4 a, 4 b, 4 c are all atmutually different distances from the outer surface of the concretestructure 100. Specifically, the plurality of second electrodes 4 a, 4b, 4 c are provided lined up in the stated order, from the outer surfaceof the concrete structure 100 inward.

Furthermore, the first electrode 3 a and the second electrode 4 a areinstalled so as to both be equidistant from the outer surface of theconcrete structure 100. The first electrode 3 b and the second electrode4 b are installed so as to both be equidistant from the outer surface ofthe concrete structure 100. The first electrode 3 c and the secondelectrode 4 c are installed so as to both be equidistant from the outersurface of the concrete structure 100.

With such first electrodes 3 a, 3 b, 3 c and second electrodes 4 a, 4 b,4 c, the first electrodes 3 a, the first electrode 3 a and the secondelectrode 4 a form a pair, the first electrode 3 b and the secondelectrode 4 b form a pair, and the first electrode 3 c and the secondelectrode 4 c form a pair.

In the present embodiment, the sensor device 1A is configured such thatthe difference in electric potential between the first electrode 3 a andthe second electrode 4 a, the difference in electric potential betweenthe first electrode 3 b and the second electrode 4 b, and the differencein electric potential between the first electrode 3 c and the secondelectrode 4 c can each be measured by a functional element (not shown).

Herein, a more detailed description of the first electrode 3 a and thesecond electrode 4 a shall now be provided. Each of the configurationsof the first electrode 3 b and the first electrode 3 c is similar to theconfiguration of the first electrode 3 a, and each of the configurationsof the second electrode 4 b and the second electrode 4 c is similar tothe configuration of the second electrode 4 a.

In the present embodiment, as illustrated in FIG. 12A, the firstelectrode 3 a is constituted of a porous body 32A having connectingholes, where adjacent holes 31A communicate with each other. The porousbody 32A is provided with a substrate 321, as well as with a conductivefilm 322 provided on the substrate 321 and constituted of a differentmaterial from that of the substrate 321.

Having the first electrode 3 a so constituted from the substrate 321 andthe conductive film 322 makes it possible to have the conductive film322 (the vicinity of the surface of the first electrode 3 a) constitutedof a metal from which it is difficult to produce a porous body. Thethickness of the conductive film 322 can also be used to adjust the holediameter of the porous body constituting the first electrode 3 a.

The constituent material of the substrate 321 of the first electrode 3 aof such description may be a material having conductivity, and, inaddition to the constituent materials of the first electrode 3 in thefirst embodiment described above (the first metallic material), aconductive ceramic can also be used.

The constituent material of the first electrode 3 in the firstembodiment described above (the first metallic material) can also beused as the constituent material of the conductive film 322.

Such a first electrode 3 a is obtained, for example, by the formation ofthe substrate 321, which is a porous body, and the formation of theconductive film 322 on the substrate 321 using electroplating or anothertechnique.

In particular, preferably, the conductive film 322 is constituted of ametallic material in which either a passivation film is formed on asurface thereof or a passivation film present on a surface thereof islost, in association with changes in the environment of the site to bemeasured.

Thereby, the difference in electric potential between the firstelectrode 3 a and the second electrode 4 a has sharp changes dependingon the presence or absence of the passivation film, which is associatedwith changes in pH of the site to be measured. For this reason, it ispossible to accurately measure whether or not the pH of the site to bemeasured is at or below a set value.

The difference in electric potential between the first electrode 3 a andthe second electrode 4 a also has sharp changes depending on the loss ofthe passivation film, which is associated with changes in the chlorideion concentration of the site to be measured. For this reason, it ispossible to accurately measure whether or not the chloride ionconcentration of the site to be measured is at or below a set value.

By contrast, as illustrated in FIG. 12B, the second electrode 4 a is,similarly with respect to the second electrode 4 in the first embodimentdescribed above, constituted of a porous body 42 having connectingholes, where adjacent holes 41 communicate with each other.

The second electrode 4 a, too, similarly with respect to the firstelectrode 3 a, may be constituted of a porous body provided with asubstrate as well as with a conductive film provided on the substrate.

According to such a sensor device 1A according to the second embodiment,it is possible to accurately detect whether or not the pHs of theinstallation environments of the first electrode 3 a and the secondelectrode 4 a, the installation environments of the first electrode 3 band the second electrode 4 b, and the installation environments of thefirst electrode 3 c and the second electrode 4 c are at or below a setvalue. It is possible to measure the respective differences in electricpotential and, therefore, to accurately detect whether or not the pH ofthe installation environments of the first electrodes 3 a, 3 b, 3 c andof the second electrodes 4 a, 4 b, 4 c is at or below a set value. Thatis, it is possible to accurately detect whether or not the pH atpositions of different depths from the outer surface of the concretestructure 100 is at or below a set value. This makes it possible todetect the speed at which the pH of the concrete 101 is changing towardbeing more acidic. For this reason, it is possible to effectivelypredict the infiltration of neutralization (or salt damage) in the depthdirection of the concrete structure 100.

The preceding is a description of the sensor device of the presentinvention, based on the depicted embodiments, but the present inventionis in no way limited thereto.

For example, the configuration of each of the parts in the sensor deviceof the present invention can be substituted with any desiredconfiguration for exerting similar functions, and any desiredconfiguration can be added.

Also, the embodiments described above are descriptions, by way ofexample, of a case where each of the first electrode and the secondelectrode is provided on the substrate, but there is no limitationthereto, and, for example, the first electrode and the second electrodemay also be provided, for example, on the outer surface of the portionof the main body of the sensor device constituted of the sealing resin.

Further, the embodiments described above are descriptions, by way ofexample, of a case where the first electrode and the second electrodeeach form the shape of a thin film, but there is no limitation thereto,and the shapes of the first electrode and the second electrode may alsoeach form, for example, a block shape, a wire shape, or the like. In theembodiments described above, the first electrode and the secondelectrode are each provided along the outer surface of the main body ofthe sensor device, but the first electrode and the second electrode mayalso each be projected out from the outer surface of the main body ofthe sensor device. In addition, the installation locations, size(relative sizes), and other aspects of the first electrode and thesecond electrode are also not limited by the embodiments describedabove, and may be as desired provided that measurement as describedabove is possible.

Also, the embodiments described above are descriptions, by way ofexample, of a case where the functional element has a CPU, an A/Dconversion circuit, and a differential amplifier circuit, but there isno limitation thereto, and, for example, a ROM, RAM, various types ofdrive circuits, and other, additional circuits may be incorporated intothe functional element.

The embodiments described above are descriptions, by way of example, ofa case where information relating to the difference in electricpotential between the first electrode and the second electrode istransmitted outside the sensor device by active tag communication bywireless transmission, but there is no limitation thereto, and, forexample, passive tag communication may be used to transmit theinformation outside the sensor device, or the information may betransmitted outside the sensor device by wire.

The embodiments described above are descriptions, by way of example, ofa case where the functional element 51, the power source 52, thetemperature sensor 53, the communication circuit 54, the antenna 55, andthe oscillator 56 are housed in the main body 2, and these elements isare, together with the first electrode 3 and the second electrode 4,embedded in the concrete structure 100, which is the object to bemeasured, but the functional element 51, the power source 52, thetemperature sensor 53, the communication circuit 54, the antenna 55, andthe oscillator 56 may also be provided outside the object to bemeasured.

The embodiments described above are descriptions, by way of example, ofa case where both the first electrode and the second electrode areconstituted of porous bodies, but the effects of the present inventioncan be achieved whenever at least one electrode or more of the firstelectrode and the second electrode is constituted of a porous body.

General Interpretation of Terms

In understanding the scope of the present invention, the term“comprising” and its derivatives, as used herein, are intended to beopen ended terms that specify the presence of the stated features,elements, components, groups, integers, and/or steps, but do not excludethe presence of other unstated features, elements, components, groups,integers and/or steps. The foregoing also applies to words havingsimilar meanings such as the terms, “including”, “having” and theirderivatives. Also, the terms “part,” “section,” “portion,” “member” or“element” when used in the singular can have the dual meaning of asingle part or a plurality of parts. Finally, terms of degree such as“substantially”, “about” and “approximately” as used herein mean areasonable amount of deviation of the modified term such that the endresult is not significantly changed. For example, these terms can beconstrued as including a deviation of at least ±5% of the modified termif this deviation would not negate the meaning of the word it modifies.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. Furthermore, the foregoing descriptions of theembodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A sensor device comprising: a first electrodebeing constituted of a porous body that has a plurality of holes in atleast the vicinity of a surface of the first electrode, an averagediameter of the plurality of the holes being in a range that allows foroccurrence of a capillary condensation effect, the first electrode beingintegrated as a one piece, unitary metallic member, the first electrodeincluding a first metallic material in which either a first passivationfilm is formed on a surface thereof or a first passivation film presenton a surface thereof is lost, in association with changes in anenvironment of the site to be measured, the first metallic materialhaving a first pH dependency with respect to at least one of formationand loss of the first passivation film; a second electrode spaced apartfrom the first electrode, the second electrode including a secondmetallic material in which either a second passivation film is formed ona surface thereof or a second passivation film present on a surfacethereof is lost, in association with changes in the environment of thesite to be measured, the second metallic material having a second pHdependency with respect to at least one of formation and loss of thesecond passivation film, the second pH dependency being different fromthe first pH dependency; and a functional element configured to measurea difference in electric potential between the first electrode and thesecond electrode, the sensor device being configured to measure a stateof a site to be measured based on the difference in electric potentialas measured by the functional element.
 2. The sensor device according toclaim 1, wherein the second electrode is constituted of a porous bodythat has a plurality of holes in at least the vicinity of a surface ofthe second electrode, and an average diameter of the plurality of theholes is in a range that allows for occurrence of a capillarycondensation effect.
 3. The sensor device according to claim 2, whereinat least one of the first electrode and the second electrode includes asubstrate and a conductive film provided on the substrate including amaterial different from that of the substrate.
 4. The sensor deviceaccording to claim 3, wherein the conductive film includes a metallicmaterial in which either a passivation film is formed on a surfacethereof or a passivation film present on a surface thereof is lost, inassociation with changes in an environment of the site to be measured.5. The sensor device according to claim 1, wherein each of the firstmetallic material and the second metallic material is iron or aniron-based alloy.
 6. The sensor device according to claim 1, wherein thefunctional element is further configured to detect whether or not pH orchloride ion concentration at the site to be measured is at or below aset value, based on the difference in electric potential between thefirst electrode and the second electrode.
 7. The sensor device accordingto claim 1, further comprising an antenna and a communication circuitconfigured to provide power to the antenna, the functional element beingfurther configured to drive and control the communication circuit. 8.The sensor device according to claim 1, wherein the first metallicmaterial of the first electrode has a first range of pHs where the firstpassivation film is formed, the second metallic material of the secondelectrode has a second range of pHs where the second passivation film isformed, the first range of pHs has a first passivation pH that is alower limit of the first range of pHs, and the second range of pHs has asecond passivation pH that is a lower limit of the second range of pHsand different from the first passivation pH.